Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm (3–5 in) range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird. They can hover in mid-air by rapidly flapping their wings 12–90 times per second (depending on the species). They are also the only group of birds able to fly backwards.[1] Their English name derives from the characteristic hum made by their rapid wing beats. They can fly at speeds exceeding 15 m/s (54 km/h, 34 mi/h).[2]

Contents

Diet and specialization for food gathering

Hummingbirds drink nectar, a sweet liquid inside certain flowers. Like bees, they are able to assess the amount of sugar in the nectar they eat; they reject flower types that produce nectar that is less than 10% sugar and prefer those whose sugar content is stronger. Nectar is a poor source of nutrients, so hummingbirds meet their needs for protein, amino acids, vitamins, minerals, etc. by preying on insects and spiders.[3]

Most hummingbirds have bills that are long and straight or nearly so, but in some species the bill shape is adapted for specialized feeding. Thornbills have short, sharp bills adapted for feeding from flowers with short corollas and piercing the bases of longer ones. The Sicklebills' extremely decurved bills are adapted to extracting nectar from the curved corollas of flowers in the family Gesneriaceae. The bill of the Fiery-tailed Awlbill has an upturned tip, as in the Avocets. The male Tooth-billed Hummingbird has barracuda-like spikes at the tip of its long, straight bill.

The two halves of a hummingbird's bill have a pronounced overlap, with the lower half (mandible) fitting tightly inside the upper half (maxilla). When hummingbirds feed on nectar, the bill is usually only opened slightly, allowing the tongue to dart out and into the interior of flowers.

Like the similar nectar-feeding sunbirds and unlike other birds, hummingbirds drink by using protrusible grooved or trough-like tongues.[4]
Hummingbirds do not spend all day flying, as the energy cost would be prohibitive; the majority of their activity consists simply of sitting or perching. Hummingbirds feed in many small meals, consuming many small invertebrates and up to twelve times their own body weight in nectar each day. They spend an average of 10–15% of their time feeding and 75–80% sitting and digesting.

Many plants pollinated by hummingbirds produce flowers in shades of red, orange, and bright pink, though the birds will take nectar from flowers of many colors. Hummingbirds can see wavelengths into the near-ultraviolet, but their flowers do not reflect these wavelengths as many insect-pollinated flowers do. This narrow color spectrum may render hummingbird-pollinated flowers relatively inconspicuous to most insects, thereby reducing nectar robbing.[6][7] Hummingbird-pollinated flowers also produce relatively weak nectar (averaging 25% sugars w/w) containing high concentrations of sucrose, whereas insect-pollinated flowers typically produce more concentrated nectars dominated by fructose and glucose.[8]

Aerodynamics of flight

Hummingbird flight has been studied intensively from an aerodynamic perspective using wind tunnels and high-speed video cameras.

Writing in Nature, the biomechanistDouglas Warrick and coworkers studied the Rufous Hummingbird, Selasphorus rufus, in a wind tunnel using particle image velocimetry techniques and investigated the lift generated on the bird's upstroke and downstroke. They concluded that their subjects produced 75% of their weight support during the downstroke and 25% during the upstroke. Many earlier studies had assumed (implicitly or explicitly) that lift was generated equally during the two phases of the wingbeat cycle, as is the case of insects of a similar size. This finding shows that hummingbirds' hovering is similar to, but distinct from, that of hovering insects such as the hawk moths.[10]

The Giant Hummingbird's wings beat at 8–10 beats per second, the wings of medium-sized hummingbirds beat about 20–25 beats per second and the smallest can reach 100 beats per second during courtship displays.

Metabolism

With the exception of insects, hummingbirds while in flight have the highest metabolism of all animals, a necessity in order to support the rapid beating of their wings. Their heart rate can reach as high as 1,260 beats per minute, a rate once measured in a Blue-throated Hummingbird.[11] They also consume more than their own weight in nectar each day, and to do so they must visit hundreds of flowers daily. Hummingbirds are continuously hours away from starving to death, and are able to store just enough energy to survive overnight.[12]

Hummingbirds are capable of slowing down their metabolism at night, or any other time food is not readily available. They enter a hibernation-like state known as torpor. During torpor, the heart rate and rate of breathing are both slowed dramatically (the heart rate to roughly 50–180 beats per minute), reducing the need for food.

The dynamic range of metabolic rates in hummingbirds[13] requires a corresponding dynamic range in kidney function.[14] The glomerulus is a cluster of capillaries in the nephrons of the kidney that removes certain substances from the blood, like a filtration mechanism. The rate at which blood is processed is called the glomerular filtration rate (GFR). Most often these fluids are reabsorbed by the kidneys. During torpor, to prevent dehydration, the GFR slows, preserving necessities for the body such as glucose, water and salts. GFR also slows when a bird is undergoing water deprivation. The interruption of GFR is a survival and physiological mechanism unique to hummingbirds.[14]

Studies of hummingbirds' metabolisms are highly relevant to the question of how a migratingRuby-throated Hummingbird can cross 800 km (500 mi) of the Gulf of Mexico on a nonstop flight. This hummingbird, like other birds preparing to migrate, stores up fat to serve as fuel, thereby augmenting its weight by as much as 100 percent and hence increasing the bird's potential flying time.[15]

Lifespan

Hummingbirds have long lifespans for organisms with such rapid metabolisms. Though many die during their first year of life, especially in the vulnerable period between hatching and leaving the nest (fledging), those that survive may live a decade or more. Among the better-known North American species, the average lifespan is 3 to 5 years. By comparison, the smaller shrews, among the smallest of all mammals, seldom live more than 2 years.[16] The longest recorded lifespan in the wild is that of a female Broad-tailed Hummingbird that was banded (ringed) as an adult at least one year old then recaptured 11 years later, making her at least 12 years old. Other longevity records for banded hummingbirds include an estimated minimum age of 10 years 1 month for a female Black-chinned similar in size to Broad-tailed, and at least 11 years 2 months for a much larger Buff-bellied Hummingbird.[17]

Range

Hummingbirds are restricted to the Americas from southern Alaska to Tierra del Fuego, including the Caribbean. The majority of species occur in tropical and subtropical Central and South America, but several species also breed in temperate climates and some hillstars even occur in alpine Andean highlands at altitudes of up to 5,200 metres (17,000 ft).[18] The greatest species richness is in humid tropical and subtropical forests of the northern Andes and adjacent foothills, but the number of species found in the Atlantic Forest, Central America or southern Mexico also far exceeds the number found in southern South America, the Caribbean islands, the United States and Canada. While fewer than 25 different species of hummingbirds have been recorded from the United States and fewer than 10 from Canada and Chile each,[19]Colombia alone has more than 160[20] and the comparably small Ecuador has about 130 species.[21]

Most hummingbirds of the U.S. and Canada migrate south in fall to spend the winter in northern Mexico or Central America. A few southern South American species also move to the tropics in the southern winter. A few species are year-round residents in the warmer coastal and interior desert regions. Among these is Anna's Hummingbird, a common resident from southern California inland to southern Arizona and north to southwestern British Columbia.

The Rufous Hummingbird is one of several species that breed in western North America and are wintering in increasing numbers in the southeastern United States, rather than in tropical Mexico. Thanks in part to artificial feeders and winter-blooming gardens, hummingbirds formerly considered doomed by faulty navigational instincts are surviving northern winters and even returning to the same gardens year after year. Individuals that survive winters in the north, however, may have altered internal navigation instincts that could be passed on to their offspring. The Rufous Hummingbird nests farther north than any other species and must tolerate temperatures below freezing on its breeding grounds. This cold hardiness enables it to survive temperatures well below freezing, provided that adequate shelter and feeders are available.

Reproduction

As far as is known, male hummingbirds do not take part in nesting. Most species build a cup-shaped nest on the branch of a tree or shrub, though a few tropical species normally attach their nests to leaves. The nest varies in size relative to species, from smaller than half of a walnut shell to several centimeters in diameter. In many hummingbird species, spider silk is used to bind the nest material together and secure the structure to its support. The unique properties of silk allow the nest to expand with the growing young. Two white eggs are laid, which, despite being the smallest of all bird eggs, are in fact large relative to the hummingbird's adult size. Incubation lasts 14 to 23 days, depending on species, ambient temperature, and female attentiveness to the nest. Their mother feeds the nestlings on small arthropods and nectar by inserting her bill into the open mouth of a nestling and regurgitating the food into its crop.

Sonation during display dives

The outer tail-feathers of male Anna's Hummingbird (Calypte anna) vibrate during display dives and produce a loud chirp. When courting, the male ascends some 30m before diving over an interested female at high speed and producing a high-pitched sound. Experiments showed that the birds could not make the sound when missing their outer tail-feathers, and that those same feathers could produce the dive-sound in a wind tunnel. The bird can sing at the same frequency as the tail-feather chirp, but its weak syrinx is not capable of the same volume. Many other species of hummingbirds also produce sounds with their wings or tail, including the wings of the Broad-tailed Hummingbird, Rufous Hummingbird, Allen's Hummingbird, Streamertail, as well as the tail of the Costa's Hummingbird and the Black-chinned Hummingbird.[23]

Systematics and evolution

In traditional taxonomy, hummingbirds are placed in the order Apodiformes, which also contains the swifts. However, some taxonomists have separated them into their own order, Trochiliformes. Hummingbirds' wing bones are hollow and fragile, making fossilization difficult and leaving their evolutionary history poorly documented. Though scientists theorize that hummingbirds originated in South America, where there is the greatest species diversity, possible ancestors of extant hummingbirds may have lived in parts of Europe to what is southern Russia today.[24]

There are between 325 and 340 species of hummingbird, depending on taxonomic viewpoint, divided into two subfamilies, the hermits (subfamily Phaethornithinae, 34 species in six genera), and the typical hummingbirds (subfamily Trochilinae, all the others). However, recent phylogenetic analyses suggest that this division is slightly inaccurate, and that there are nine major clades of hummingbirds: the topazes and jacobins, the hermits, the mangoes, the coquettes, the brilliants, the Giant Hummingbird (Patagona gigas), the mountain-gems, the bees, and the emeralds.[25] The topazes and jacobins combined have the oldest split with the rest of the hummingbirds. The hummingbird family has the second most species of any bird family on Earth (after the tyrant flycatchers).

Fossil hummingbirds are known from the Pleistocene of Brazil and the Bahamas; however, neither has yet been scientifically described, and there are fossils and subfossils of a few extant species known. Until recently, older fossils had not been securely identifiable as those of hummingbirds.

In 2004, Dr. Gerald Mayr of the Senckenberg Museum in Frankfurt am Main identified two 30-million-year-old hummingbird fossils and published his results in Science.[26] The fossils of this primitive hummingbird species, named Eurotrochilus inexpectatus ("unexpected European hummingbird"), had been sitting in a museum drawer in Stuttgart; they had been unearthed in a clay pit at Wiesloch–Frauenweiler, south of Heidelberg, Germany and, because it was assumed that hummingbirds never occurred outside the Americas, were not recognized to be hummingbirds until Mayr took a closer look at them.

Fossils of birds not clearly assignable to either hummingbirds or a related, extinct family, the Jungornithidae, have been found at the Messel pit and in the Caucasus, dating from 40–35 mya; this indicates that the split between these two lineages indeed occurred at that date. The areas where these early fossils have been found had a climate quite similar to the northern Caribbean or southernmost China during that time. The biggest remaining mystery at the present time is what happened to hummingbirds in the roughly 25 million years between the primitive Eurotrochilus and the modern fossils. The astounding morphological adaptations, the decrease in size, and the dispersal to the Americas and extinction in Eurasia all occurred during this timespan. DNA-DNA hybridization results [27] suggest that the main radiation of South American hummingbirds at least partly took place in the Miocene, some 12–13 mya, during the uplifting of the northern Andes.

Wing structure and colours

Many of the Hummingbird species have bright plumage with exotic colouration. In many species, the coloring does not come from pigmentation in the feather structure, but instead from prism-like cells within the top layers of the feathers. When light hits these cells, it is split into wavelengths that reflect to the observer in varying degrees of intensity. The Hummingbird wing structure acts as a diffraction grating. The result is that, merely by shifting position, a muted-looking bird will suddenly become fiery red or vivid green.[28] However, not all hummingbird colors are due to the prism feather structure. The rusty browns of Allen's and Rufous Hummingbirds come from pigmentation. Iridescent hummingbird colors actually result from a combination of refraction and pigmentation, since the diffraction structures themselves are made of melanin, a pigment.[29]

Lists of species and genera

Feeders and artificial nectar

Hummingbirds will also take sugar-water from bird feeders. Such feeders allow people to observe and enjoy hummingbirds up close while providing the birds with a reliable source of energy, especially when flower blossoms are less abundant.

White granulated sugar is the best sweetener to use in hummingbird feeders. A ratio of 1 cup sugar to 4 cups water is a common recipe.[30] Boiling and then cooling this mixture before use has been recommended to help deter the growth of bacteria and yeasts. Powdered sugars contain corn starch as an anti-caking agent; this additive can contribute to premature fermentation of the solution. Brown, turbinado, and "raw" sugars contain iron, which can be deadly to hummingbirds if consumed over long periods.[31]Honey is made by bees from the nectar of flowers, but it is not good to use in feeders because when it is diluted with water, microorganisms easily grow in it, causing it to spoil rapidly.[32][33]

Red food dye is often added to homemade solutions. Commercial products sold as "instant nectar" or "hummingbird food" may also contain preservatives and/or artificial flavors as well as dyes. The long-term effects of these additives on hummingbirds have not been studied, but studies on laboratory animals indicate the potential to cause disease and premature mortality at high consumption rates.[34] Although some commercial products contain small amounts of nutritional additives, hummingbirds obtain all necessary nutrients from the insects they eat. This renders the added nutrients unnecessary.[22]

Other animals also visit hummingbird feeders. Bees, wasps, and ants are attracted to the sugar-water and may crawl into the feeder, where they may become trapped and drown. Orioles, woodpeckers, bananaquits, and other larger animals are known to drink from hummingbird feeders, sometimes tipping them and draining the liquid.[35] In the southwestern United States, two species of nectar-drinking bats (Leptonycteris yerbabuenae and Choeronycteris mexicana) visit hummingbird feeders to supplement their natural diet of nectar and pollen from saguaro cacti and agaves.[36]

In myth and culture

Aztecs wore hummingbird talismans, the talismans being representations as well as actual hummingbird fetishes formed from parts of real hummingbirds: emblematic for their vigor, energy, and propensity to do work along with their sharp beaks that mimic instruments of weaponry, bloodletting, penetration, and intimacy. Hummingbird talismans were prized as drawing sexual potency, energy, vigor, and skill at arms and warfare to the wearer.[37]

The Aztec god Huitzilopochtli is often depicted as a hummingbird. The Nahuatl word huitzil (hummingbird) is an onomatopoeic word derived from the sounds of the hummingbird's wing-beats and zooming flight.